boiler tubular air pre-heater - taph

11.00.00.00 PROCEDURE FOR TUBULAR AIR HEATER

11.10.00.00 GENERAL

11.11.00.00 Tubular air heaters are broadly classified based on

a) Type of flowb) Tube arrangement

11.11.10.00 Classification based on type of flow

Fluid medium involved shall be air (heated medium) and boiler flue gas (heating medium). Out of these mediums, one is made to pass through the tubes and the other is made to pass over the tubes. Generally, hot flue gas shall pass through the tubes, while air shall pass over the tubes. This type of air heater shall be called as “Gas through air heaters”.

The other type shall be “Air through air heaters” that means air shall pass through tubes and gas shall pass over the tubes. This type of tubular air heater is also equally popular.

11.11.11.00 Applicability of the air heater type

As per CVPL’s practice, Gas through air heaters are adopted for steam generator capacity up to 70 t/h

Above 70 t/h steam generation, Air through air heaters are being adopted.

11.11.20.00 Classification based on tube arrangement

Tubular air heaters shall be classified as “ In line arrangement” and “ Staggered arrangement”, based on the tube arrangement.

Normally staggered arrangement is preferred due to its compactness and high heat transfer rate.

In staggered arrangement, multi rows of tubes shall be arranged in planes perpendicular to the airflow direction. Tubes in one plane shall be spaced at a pitch of “air side pitching” (also called transverse pitching). Next row of tubes shall be placed along the airflow direction, at a distance of half the “gas side pitching” (also called longitudinal pitching) but staggered to previous row of tubes by half of airside pitching. Third row shall be the repeat of 1st row and fourth row shall be the repeat of 2nd row and so on. This arrangement shall be repeated to get the total number of tubes per pass (as furnished in Thermal data)

Title Tubular Air heater Page Date Rev.Sub title Tubular Air heater 1 of 18 Aug. 2004 00

The dimension of the air heater perpendicular to airflow direction shall be width of the air heater and dimension of the air heater along the airflow direction shall be air heater depth.

Number of tubes in 1st row shall be called as “airside tubes” (NA) as defined in Thermal data.

These tubes are connected to tube sheets on both ends. The distance between the tube sheets shall be called pass height (HP)

11.11.30.00 Definition of terms

The following are the major inputs from Thermal data and their definitions as described below.

a) Number of tubes air side (NA)

Number of tubes in a single row (usually first row) perpendicular to air flow direction.

b) Air side pitching (ST) Centre line distance between the tubes of airside tubes as defined above.

c) Gas side pitching (SL) Centre line distance between the tubes of same plane along the air flow direction.

d) Tubes per pass (NT) Total number of tubes per air pass. Generally, there shall be one air pass per block. Under such conditions, NT

shall be defined as number of tubes per block

e) Width (W) This is a dimension reached by multiplying the air side tubes by air side pitchingi.e., W = NA x ST

This is also the casing inner dimension.f) Tube length (L) This is overall length of the air heater

tube. Pass height (HP) shall be arrived from Tube length (L) as indicated below

HP = L- (tP1+ tP2+ XE+ XL+ t FP+ B)

WheretP1

tP2 Thickness of the connecting tube sheets

XE Tube extension on gas entry side

XL Tube extension on gas leaving side

tFP Thickness of the flange plateB Height of expansion bellows)


11.20.00.00 Input data

11.20.10.00 Thermal input data

i) Type of flowii) Tube arrangementiii) Tube O.D d mmiv) Tube thickness t mmv) Air side pitch (spacing perpendicular to air

flow direction)ST mm

vi) Gas side pitch (spacing along the air flow direction)

SL mm

vii) No. of tubes – Air side (Tubes in a single plane perpendicular to air flow direction

NA

viii) No. of gas pass PG

ix) No. of air pass PA

x) No. of tubes per pass NT

xi) Tube length per pass Lxii) Width of Air heater W mxiii) Air quantity WA kg/hrxiv) Air heater- Air inlet temperature t in Cxv) Air heater- Air outlet temperature t out Cxvi) Air heater support frame details

11.20.20.00 In-Built data

Number of tube sections per pass NB

Space between tube sections SB

Recommended cold air velocity Vca 12 m/secRecommended hot air velocity Vha 20 m/sec

11.30.00.00 Gas through tube air heater

Gas through tubes air heater is generally vertical tube air heater with gas flowing vertically through the tubes and air passing over tubes and move horizontally.

11.31.10.00 Air heater supporting arrangement

Vertical tube (gas through) air heaters shall be provided with support frames to transfer the air heater load to beams and columns.

As per CVL practice, there will two blocks of air heater. The block where the hot flue gas from economizer entering shall be termed as AIR HEATER-I and the other block as AIR HEATER-II.

Each block shall be provided with two frames as explained below.

Frame means a rectangular structure made out of steel


channels/beams encompassing the air heater dimensions.

There shall be channels (running along the minor axis of the frame) at regular intervals to act as stiffeners to the air heater.

i) Bottom support frame

This frame shall be located at the bottom of the AIR HEATER-I. This frame ultimately transfers the entire air heater load to beams/ columns

ii) Middle frame

Normally there will be two middle frames; one located at the top of the AIRHEATER-I and other is located at the bottom of the AIRHEATER-II. These two frames are normally of same size.

iii) Top frame

This frame shall be located at the top of AIRHEATER-II

The sizes of the channels and beams and major dimensions of the frame shall be provided by the structural engg. Group. However the spacing between the stiffener channels shall not be indicated.

11.31.11.00 Bottom support frame

i) Main members

a) Bottom support frame consists of two main members, normally of box type or I beams running along the major axis of the air heater block.

b) The size of member shall be from structural output.

c) These are the members to transfer the weight of the air heater to load points.

d) In case of box members made out of channels, the cross sectional dimensions of the member shall be as follows.Horizontal dimension (h) h = 2 x (Channel flange width +1) . mmVertical dimension (b)b= Channel height mm

e) In case of members by I beam, horizontal dimension shall be the flange width and vertical dimension shall be the beam height.


f) Member length (x) (along the major axis of the AH)x= span between the load points ( S1) + 350 mm The value of S1 shall be indicated in structural output.

g) Spacing (Center to center distance) between main members (y)(Along the minor axis of the AH)Y = W +hWhere W =Air heater width as defined in 11.11.30.00 (e)

h) Base plateThe main members shall be welded with a base plate of 350 mm square (in most of the cases) on each end such a way that the center of the base plate shall be located 175 mm from each end of the main member.

The base plate shall be usually 16 mm thick.

ii) Cross membersa) There shall be two cross members running along the minor axis of

the air heater connecting the main members.

b) Mostly these cross members shall be channels and size of the channel shall be from structural output.

c) If the size of the cross member channel is less than the main member channel, adopt the size of the main member channel itself for cross member also.

d) These cross member channels shall be placed at equidistance from each end of the main member.

e) The cross member channels shall face the opposite directions

f). The inner dimension (S2) between the cross members shall be obtained structural output.

g) The centerline distance between the cross members shall be equal to S2 + Channel flange width.

iii) Stiffener members

a) The stiffener members also like cross members running along the minor axis of the air heater connecting the main members

b) Normally the stiffener member shall be of 150 mm channels

c) The extreme stiffener members (adjacent to cross members) shall face the same direction of cross member.


d) Adjacent stiffener members in between the extreme stiffeners shall face the opposite direction to each other.

e) Number of stiffener (NR)

NR =(Number of tube sections –1)i.e., NR = (NB –1)

e) Stiffener members’ size and arrangement shall be identical for all the frames.

f) The spacing between the stiffener members shall be obtained as explained in clause 11.32.11.30

11.31.12.00 Middle frame (for both AH-I top side and AH-II bottom side)

i) Main members

a) Middle frame consists of two main members, normally of channel type running along the major axis of the air heater block.


c) If the size of the member is less than channel 300, adopt channel 300 to accommodate sufficient inter block space.

d) Member length (x) (along the major axis of the AH)x= inner dimension from structural output + channel flange width.

g) Spacing (Center to center distance) between main members (y)(Along the minor axis of the AH)

Y = W +Channel flange widthWhere W =Air heater width as defined in 11.11.30.00 (e)




c) The cross member channels shall face the opposite directions




11.31.13.00 Top frame

i) Main members

a) Top frame consists of two main members, normally of channel type running along the major axis of the air heater block.


c) Member length (x) (along the major axis of the AH)x= inner dimension from structural output + channel flange width.

d) Spacing (Center to center distance) between main members (y)(along the minor axis of the AH)

Y = W +Channel flange widthWhere W =Air heater width as defined in 11.11.30.00 (e)




c) The cross member channels shall face the opposite directions



11.32.00.00 Tubing arrangement in a pass

11.32.10.00 Number of sections per pass (NB)

Approximate number of tubes deep along the air flow direction (per pass)

11.1

Tentative number of section in a pass shall be

11.2


Number of section per pass shall be

NB = N +0. 5 11.3

Round off the value of NB to the nearest integer.

11.32.11.00 Tubing arrangement in section

Approximate number of tubes per section (T)

11.4

11.32.11.10 Number of tubes deep per section (odd number of row along the air flow direction) nd.1

11.5

Round off the value of nd.1 to the nearest integer on higher side

11.32.11.20 Number of tubes deep per section (even number of row along the air flow direction) nd.2

nd.2 = nd.1-1 11.6

11.32.11.30 Check

T1= NA x nd.1 + (NA-1) nd.2 11.7

Where NA is the mumers of air side tubes

T1 should be T

Otherwise, choose

nd.2 = nd.1

Now T1 = nd.1 x (2NA-1)

It is considered that atleast two sections shall have same tubing arrangement. For example, in a three section arrangement, section-1 and section- 3 shall be with same tube arrangement. Accordingly, number of tubes in section -2 shall be

T2 = NT – 2T1 11.8


Number of tubes deep in other section shall be

N d.1=n d.2= 11.9

11.32.11.20 Section depth (DS)

If n d.2= n d1 11.10

If n d.2= n d.1 - 1 11.11

11.32.11.30 Spacing between the stiffener member (SB)

SB = DS +(65+65+75) 11.12Where 65 mm is the clearence between tube and flange75 mm is the flange width of the stiffener member (150 channel)

However this dimension may be adjusted to meet the inner dimension of the casing,

11.33.00.00 Dimensions

11.33.10.00 Air heater (pass) dimensions

Air heater dimensions refer to the casing inner dimensions. These dimensions shall also be referred as pass dimensions

11.33.11.00 Air heater width (W)

This is the dimension between the inner distance between the support, in a direction perpendicular to air flow direction. This dimension shall be directly taken from Thermal output.

11.33.12.00 Air heater depth (D)

This is the dimension between the inner distance between the support, in a direction paralel to air flow direction. This dimension shall be arrived as indicated below

D= (NB x DS) + (NB –1)*(SB-DS) + SL 11.13

11.33.13.00 Pass height (HP)

HP = L- (tP1+ tP2+ XE+ XL+ t FP+ B) 11.14


tP1

tP2 Thickness of the connecting tube sheets

XE Tube extension on gas entry side (25 mm)XL Tube extension on gas leaving side (15 mm)tFP Thickness of the flange plate (normally 16 mm)B Height of expansion bellows (42 mm)

11.33.20.00 Inter pass dimension (P)

The inter pass dimension in the space between two blocks. This shall be arrived as given below.

P = 2 x (flange height of middle frame cross members) + man hole height (450 mm)

11.34.00.00 Tube sheet dimensions

11.34.10.00 Length (A)

Dimension of the tube sheet parallel to airflow direction shall be defined as Length.

11.34.11.00 Length for AH-I bottom tube sheet (A1)

A1 = D +2FC +20 11.15

Where D Air heater depth defined in clause 11.33.13.00FC Flange width of the cross member (Channel) in the bottom

frame

11.34.12.00 Length for AHI top tube sheet (A2)

A2 = D +(2 x 150) 11.16

Where D Air heater depth defined in clause 11.33.13.00

11.34.13.00 Length for AH-II bottom tube sheet (A3)

A3 = D +2FC +20 11.17

Where D Air heater depth defined in clause 11.33.13.00FC Flange width of the cross member (Channel) in the middle

frame


11.34.14.00 Length for AH-II top tube sheet (A4)

A4 = D +(2 x 150) 11.18

11.34.20.00 Width (B)

Dimension of the tube sheet perpendicular to air flow direction shall be defined as width

11.34.21.00 Width for AH-I bottom tube sheet (B1)

B1= W + (2 x h) +20 11.19

WhereW Air heater widthh Horizontal dimension of the bottom frame main member.

11.34.22.00 Width for AH-I top tube sheet (B2)

B2= W + (2 x 150) 11.20

11.34.23.00 Width for AH-II bottom tube sheet (B3)

B3 = W +2FM +20 11.21

Where W Air heater Width FM Flange width of the main member (Channel) in the middle frame

11.34.24.00 Width for AH-II top tube sheet (B4)

B4= W + (2 x 150) 11.22

11.34.30.00 End clearances (C, D, E, F)

The distance between the tube sheet edge and corner tubes of dissimilar rows at the air entry side section and air leaving section shall be indicated as end clearances. This clearances shall be marked along width and depth directions

11.34.31.00 Clearance C

Along the depth direction, this is distance between the tube edge and the corner tube of the first row.

11.34.31.10 Clearance for AH-I bottom tube sheet (C1)

C1 = FC +(0.5 x SL) +10 11.23


WhereFC Flange width of cross member of the bottom frame.SL Gas side pitching

11.34.31.20 Clearance for AH-I top tube sheet (C2)

C2 = (0.5 x SL) +150 11.24

11.34.31.30 Clearance for AH-II bottom tube sheet (C3)

C3 = FC +(0.5 x SL) +10 11.25

WhereFC Flange width of cross member of the middle frame.SL Gas side pitching

11.34.31.40 Clearance for AH-II top tube sheet (C4)

C4 = (0.5 x SL) +150 11.26

11.34.32.00 Clearance D

Along the depth direction, this is distance between the tube edge and the corner tube of the second row.

11.34.32.10 Clearance for AH-I bottom tube sheet (D1)

D1 = C1 +(0.5 x SL) 11.27

11.34.32.20 Clearance for AH-I top tube sheet (D2)

D2 = C2 +(0.5 x SL) 11.28

11.34.32.30 Clearance for AH-II bottom tube sheet (D3)

D3 = C3 +(0.5 x SL) 11.29

11.34.32.40 Clearance for AH-II top tube sheet (D4)

D4 = C4 +(0.5 x SL) 11.30

11.34.33.00 Clearance E

Along the width direction, this is distance between the tube edge and the corner tube of the first column.


11.34.33.10 Clearance for AH-I bottom tube sheet (E1)

E1 = h + (0.5 x ST) +10 11.31

Whereh Horizontal dimension of the main memberST Air side pitching

11.34.33.20 Clearance for AH-I top tube sheet (E2)

E2 = (0.5 x ST) +150 11.32

11.34.33.30 Clearance for AH-II bottom tube sheet (E3)

E3 = FM + (0.5 x ST) +10 11.33

WhereFM Flange width of the main memberST Air side pitching

11.34.33.40 Clearance for AH-II top tube sheet (E4)

E4 = (0.5 x ST) +150 11.34

11.34.34.00 Clearance F

Along the width direction, this is distance between the tube edge and the corner tube of the second column.

11.34.34.10 Clearance for AH-I bottom tube sheet (F1)

F1 + E1 + (0.5 x ST) 11.35

11.34.34.20 Clearance for AH-I top tube sheet (F2)

F2 + E2 + (0.5 x ST) 11.36

11.34.34.30 Clearance for AH-II bottom tube sheet (F3)

F3 + E3 + (0.5 x ST) 11.37

11.34.34.40 Clearance for AH-I bottom tube sheet (F4)

F4 + E4 + (0.5 x ST) 11.38


11.35.00.00 Tube sheet thickness (tP)

Unit capacity t/h

Tube sheet thickness -tP (mm)Bottom sheet Middle sheet Top sheet

Up to 20 16 10 1021 to 40 25 20 1641 to 70 30 25 16

11.40.00.00 Air through tube air heater

Air through tubes air heater is horizontal tube air heater with air flowing through the tubes horizontally and gas passing over tubes and move vertically.

11.40.10.00 Stiffening arrangement

In this arrangement, no stiffener channels / angles are necessary. However, if the tube length is more (say more than 4 m), then it is advisable to introduce one more tube sheet in between the tube sheets to avoid sagging of tubes.

Accordingly, there shall not be any tube bank arrangement.

11.40.20.00 Tubing arrangement in single pass

11.40.21.00 Number of tubes wide per pass (odd number of row perpendicular to the air flow direction) nw.1

11.40

Round off the value of nw.1 to the nearest integer on higher side

11.40.22.00 Number of tubes wide per pass (even number of row perpendicular to the air flow direction) nw.2

nw.2 =nw.1-1 11.41

11.40.23.00 Check

NT= NA x nw.1 +(NA-1) (nw.2-1) 11.42


11.40.30.00 Air heater casing (inside) dimensions

i) Width (W)

This is the horizontal dimension of the casing perpendicular to the air flow direction

W = (NA –1) ST + 2 Se 11.43

ii) Depth (D)

This is the horizontal dimension along the air flow path

D = L 11.44

iii) Height (H)

This is the vertical dimension of the casing perpendicular to the air flow direction

H =[(nw.1-1) SL] +2Se 11.45

11.40.40.00 Tube sheet thickness (tP)

For steam generation capacity 70 to 90 t/h, 20 mm for both sheets shall be adopted.

Above 90 t/h capacity, 25 mm thickness shall be used for both sheets

11.40.50.00 Air duct sizing

11.40.51.00 Air heater inlet transition duct (cold air duct)

The transition duct between the cold air duct (air duct from F.D.Fan outlet) and the air heater inlet shall be a tapered duct (tapered both in width and depth direction of the air heater). Stating otherwise, one side of the transition duct shall match the air heater dimensions and the other end shall match the cold air duct dimensions. Centre line of the air heater and transition shall be in the same line. The cold air duct dimensions shall be arrived as described below

Note: The angle of transition preferably shall not more than 25º

11.40.51.10 Flow area required (Ac)

m2 11.46


11.40.51.20 Duct dimensions

i) Duct width (WD)

mm 11.47

Round off the value of WD to the nearest 10 mm on higher side

ii) Duct depth (DD)

mm 11.48

Round off the value of DD to the nearest 10 mm on higher side

WhereWD Duct width (Duct dimension along the air heater width

direction)DD Duct depth (Duct dimension along the air heater depth

direction)

11.40.52.00 Air heater outlet transition duct (Hot air duct)

11.4052.10 Case-I: Straight tapering (tapering on both direction) duct

The transition duct between the hot air duct and the air heater outlet shall be a tapered duct (tapered both in width and depth direction of the air heater). Stating otherwise, one side of the transition duct shall match the air heater dimensions and the other end shall match the hot air duct dimensions. Centre line of the air heater and transition duct shall be in the same line. The hot air duct dimensions shall be arrived as described below

Note: The angle of transition l preferably shall not more than 25º

11.40.52.11 Flow area required (AH)

m2 11.49


i) Duct width (WD)

mm 11.50

Round off the value of WD to the nearest 10 mm on higher side


ii) Duct depth (DD)

mm 11.51


11.4052.10 Case-II: 90 bend (tapering on one direction) duct

11.40.52.11 Flow area required (AH)

The transition duct between the hot air duct and the air heater outlet shall be a tapered duct (tapered in one dimension of the air heater only) One dimension of the air heater shall be retained as hot air duct dimension while other dimension of the air heater is tapered to match the duct flow area requirement. The hot air duct opening shall be located 90 to the air heater outlet such that the hot air outlet duct shall be mostly down flow vertical duct. Normally air heater width (dimension perpendicular to air flow) shall be retained as duct dimension and the other dimension shall be tapered.)

m2 11.52


i) Duct width (WD)

Duct width shall be same as that of air heater width

mm

ii) Duct depth (DD)

mm 11.53


11.40.51.00 Air heater intermediate duct (hot air duct)

The intermediate duct between the two air heater blocks shall be a “C shaped“ duct, both end ducts are connecting the air heater blocks. Accordingly, the opening dimensions of this end duct shall match the air heater dimensions. A small rectangular middle duct shall connect these two end ducts. Width of all these ducts shall be same as that of air heater width. Depth of the middle duct alone shall be sized to meet the flow area requirement.


11.40.51.10 Flow area required (AM)

m2 11.54

Where

º C 11.55


i) Duct width (WD)

Duct width shall be same as that of air heater width

mm

ii) Duct depth (DD)

mm 11.56


However the duct depth shall not be less than 900 mm to accommodate the duct manhole.


boiler tubular air pre-heater - taph

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